It is known to produce injection-molded parts in such a way that after treatment is required in the region of the sprue position of the injection-molded parts and/or the dividing plane of the die halves. Respective methods and associated apparatuses are described among other things in U.S. Pat. No. 2,883,704, U.S. Pat. No. 3,070,843 and Japanese Patent Publication No. JP OS 48-72258.
Furthermore, molding apparatuses are known for producing sprue-free molded parts as a consequence of precise surface contact of a pair of mold parts which form a mold cavity. A molding apparatus is known from U.S. Pat. No. 3,121,918, in which a chamber is arranged on an upper surface of the bottom heating plate. An upper surface of the chamber is covered by membrane. A heat-proof fluid is filled into the chamber and held under pressure. The membrane will be deflected by the pressurized fluid and will therefore maintain very tight contact between the adjacent molds. One or several cavities can be provided in this case in the molding apparatus for producing sprue-free parts. In accordance with the teachings of this specification, the production of sprue-free parts is not possible when many cavities are provided in the molding apparatus.
In order to remedy this problem, German Patent Publication No. DE 31 07 865 A describes a molding apparatus for producing a plurality of plastic parts which are formed in a sprue-free or fin-free manner and in which the thermosetting material emerging from the injection nozzle is divided among several adjacently arranged mold cavities. The molding apparatus comprises an upper holding element for fixing at least one upper mold with a channel in which a moldable material is introduced and a bottom holding element for fixing at least one bottom mold with a cavity, with a specific quantity of a fluid being filled between a bottom surface of the bottom mold and the bottom holding element. As a result, the bottom mold floats on the fluid and is movable along the bottom holding element. After producing a surface contact between the bottom surface of the upper mold and the upper surface of the bottom mold, precise mutual contact is obtained as a result of the backpressure of the fluid. A plurality of sprue-free parts in adjacently arranged mold cavities can be injection-molded simultaneously in this manner.
It is disadvantageous in all these apparatuses, however, that all mold cavities need to be arranged adjacent to one another.
A mold with successively arranged mold cavities is known from European Patent Publication No. EP 68 615 B, with the mold cavities being separated from one another after the closing of the mold. The injection, therefore, needs to be performed during a partly open mold. Fins can therefore not be prevented. In particular, complex measures need to be provided for removing the excess material.
Japanese Patent Publication No. JP 7-314499 A shows a mold in which the mold cavity is outwardly opened in order to ensure secure filling of the mold cavity without the risk of the escape of excess material.